Blood pressure is one index for analyzing cardiovascular disease. Performing a risk analysis for cardiovascular disease based on blood pressure is effective in preventing cardiovascular-related conditions such as stroke, heart failure, and myocardial infarction. In particular, morning hypertension, in which the blood pressure rises in the early morning, is related to heart disease, stroke, and the like. Furthermore, among morning hypertension symptoms, the symptom called “morning surge”, in which the blood pressure rapidly rises within one hour to one and a half hours after waking up, has been found to have a causal relationship with stroke. Accordingly, understanding the interrelationship between time (lifestyle) and changes in blood pressure is useful in risk analysis for cardiovascular-related conditions. It is therefore necessary to continuously measure blood pressure over a long period of time.
Furthermore, recent study results have shown that home blood pressure, which is blood pressure measured at home, is more effective in the prevention, diagnosis, treatment, and so on of cardiovascular-related conditions than blood pressure measured at a hospital or during a health examination (casual blood pressure). Accordingly, sphygmomanometers for home use have become widely prevalent, and home blood pressure values have started to become used in diagnoses.
In order to improve the measurement precision of sphygmomanometers, JP H7-51233A (called “Patent Literature 1” hereinafter) discloses an invention in which processing for correcting error in a measurement value that is dependent on the characteristics of the pressure sensor for blood pressure measurement is performed in the production stage of the electronic sphygmomanometer.
JP H2-19133A (called “Patent Literature 2” hereinafter) and U.S. Pat. No. 7,594,892 (called “Patent Literature 3” hereinafter) disclose techniques for improving the reliability of blood pressure measurement values using two pressure sensors.
According to the electronic sphygmomanometer disclosed in Patent Literature 1, the correction regarding the pressure sensor is performed based on differences in the characteristics of the individuals electronic sphygmomanometers in the electronic sphygmomanometer production stage; however, unlike a sphygmomanometer used in a medical facility such as a hospital, a sphygmomanometer for home use is generally not periodically corrected after purchase, except for in certain situations such as a malfunction.
For example, even if the pressure sensor output, which is of utmost importance in blood pressure measurement, deviates beyond a specified tolerance margin, there is no way to know that this has happened, and therefore it is not clear whether blood pressure measurement values are correct. For this reason, even if there is a large difference between a blood pressure measurement value and the normal blood pressure measurement value or the casual blood pressure measurement value, it is not clear whether the blood pressure values are actually different, or the blood pressure values are different due to error in the pressure sensor of the sphygmomanometer, thus causing concern on the part of the user.
Meanwhile, some sphygmomanometers for medical facilities include two pressure sensors, and pressure is monitored based on the output of these pressure sensors. However, the functions of these two pressure sensors are used for different purposes in such sphygmomanometers. That is, the blood pressure is calculated using cuff pressure information obtained by one of the pressure sensors, and abnormality detection is performed based on the output of the other pressure sensor.
Specifically, an abnormality is detected if the pressure value detected by the pressure sensor greatly exceeds 300 mmHg, for example. In this case, safety is ensured by stopping the pump and releasing the valve. Accordingly, the other pressure sensor is applied as a safety measure specified in the Japanese medical standard IEC 60601-2-30, and does not guarantee the precision of the one pressure sensor used for blood pressure measurement.
In light of this, it is necessary for the precision of the one pressure sensor, which is used for detecting blood pressures, to be guaranteed by that pressure sensor itself. There is thus a demand for a high-precision pressure sensor that is not influenced by external disturbances such as temperature changes, that changes little over time, and that is inexpensive. Furthermore, providing two pressure sensors that perform different functions means that the malfunction rate of the sphygmomanometer due to malfunctions in the pressure sensors will simply be double the malfunction rate of a sphygmomanometer that has only one pressure sensor.
Meanwhile, a pressure sensor used in an electronic sphygmomanometer measures pressures of fluids, liquids, and so on using a pressure-sensitive element via a diaphragm (a stainless steel diaphragm, a silicon diaphragm, or the like), converts the measurement into an electric signal, and outputs the signal.
For example, in the case of a diffused piezoresistive semiconductor pressure sensor, a semiconductor strain gauge is provided on the surface of the diaphragm, and a change in electrical resistance caused by a piezoresistance effect occurring when the diaphragm deforms due to an outside force (a pressure) is converted into an electric signal.
Meanwhile, with an electrostatic capacitance pressure sensor, a capacitor is formed by opposing a glass fixed electrode and a silicon mobile electrode, and a change in electrostatic capacitance produced when the mobile electrode deforms due to an outside force (a pressure) is converted into an electric signal.
The reliability of blood pressure measurement values is maintained because only an outside force (a pressure) that is to be measured is applied to the pressure sensor. However, because the amount of deformation in the diaphragm, the amount of deformation in the mobile electrode, and so on are on the order of microns, such pressure sensors are extremely susceptible to extraneous outside stress, and it is therefore necessary to carefully consider the peripheral structure of the pressure sensor. Specific peripheral structures for a pressure sensor, however, are neither disclosed nor considered in Patent Literature 1 through 3.